SUMMARY OF WORK We have shown previously that rapamycin (RAP) inactivates p70 S6 kinase (p70S6K) in cardiac myocytes, thereby inhibiting alpha1- adrenergic-mediated cardiac myocyte hypertrophy. The purpose of these experiments was to determine whether basal or activated mitogen-activated protein kinases play a role in phenylephrine-stimulated cardiac myocyte hypertrophy. Signal transduction via the mitogen activated protein kinases (MAPK) has been implicated in cardiac myocyte hypertrophy, but the nature of its role is controversial. We examined the relative roles of agonist-stimulated increases in MAPK activity and basal levels of MAPK activity in the subsequent hypertrophic growth of neonatal rat cardiac myocytes. Serum-free myocyte cultures were stimulated with phenylephrine (PE) in the presence or absence of selected doses of PD98059, a specific inhibitor of the immediate upstream activator of MAPK, MEK. PE stimulated a transient (3-5-fold after 5 min) and a sustained (1.5-fold after 18 hours) elevation in MAPK activity, both of which were completely prevented by 10 muMPD98059. 50 muM (but not 10 muM) PD98059 reduced MAPK activity in Pretreated myocytes to levels below those in vehicle-treated cells. These actions of PD98059 were sustained for at least 72 hours. PE stimulated increases in the total protein/DNA ratio (1.3-fold), RNA content (1.4-fold), incorporation of labeled phenylalanine (14C-Phe; 2-fold), endogenous ANF mRNA (2-fold) and sarcomere organization. 10 muM PD98059 had no significant effects on the PE-induced increases in protein/DNA ratio, RNA content, incorporation of 14C-Phe, or sarcomere organization, but did suppress the increase in ANF mRNA level. In contrast, 50 muM PD98059 markedly inhibited the PE-stimulated increases in each of the aforementioned measures except sarcomere organization. It is concluded that neither the early transient, nor the sustained elevation in MAPK activity is required for PE-stimulated hypertrophy of cardiac myocytes, but that a basal level of MAPK activity plays a permissive role in hypertrophy mediated via the [unreadable]1-adrenergic receptors. Induction of ANF gene expression by PE, on the other hand, requires an elevation in MAPK activity in neonatal rat cardiac myocytes.